Cellular telephone has become more popular than anyone imagined. Nearly a hundred million people around the world are now connected to cellular mobile telephone networks. Deregulation, competition, marketing and better technology are driving cellular telephone's phenomenal growth. Even in relatively mature markets, annual growth rate for cellular telephone service can be 60% or more. New mobile networks are being opened all the time. New operators are competing aggressively to capture the consumer market for mobile telephones, positioning the mobile telephone as a genuine, and much more convenient, alternative to owning a normally wired telephone.
As existing cellular systems have continued to grow, they have run into severe capacity problems. There are not enough cellular radio channels to service all of the people who wish to operate and use cellular telephones. Existing cellular system operators are desperate for additional channels they can use to better service their expanding customer base, and new operators wishing to offer competing cellular or other mobile radio services have been clamoring for their own cellular frequency allocations.
In response to this need, the Federal Communications Commission ("FCC")has recently reallocated, for cellular telephone (and other) uses, part of the 18 MHz of radio spectrum previously allocated to the "Specialized Mobile Radio" (SMR) service. Traditionally, SMR frequencies have been operated by small entrepreneurial companies that rent out air time to third parties--usually at a flat monthly fee plus access charges to the public telephone network. To make more efficient use of this spectrum space, the FCC will auction off part of the SMR band to cellular telephone operators and other services. The resulting cellular service, sometimes called "Down Banded Cellular" (DBC) because the SMR band frequencies are below the standard cellular telephone frequencies, allows existing and new cellular telephone operators to expand cellular operations, and also gives present and former SMR operators the opportunity to "go cellular."
Downbanded cellular operators generally want to design and operate systems that are compatible with existing cellular radio equipment and standards. Providing this compatibility has important advantages. By allowing the same cellular telephone unit to access both downbanded cellular channels and conventional cellular channels, downbanded cellular frequencies can act as extensions to conventional cellular frequencies. Calls can be shifted between downbanded cellular frequencies and conventional cellular frequencies based on least cost routing or other techniques--maximizing capacity and making most efficient use of available spectrum space. In addition, users can use cellular telephone units with any cellular or downbanded cellular system that has a "roaming" agreement with a "home" cellular service provider. Cellular telephones can roam from one type of system to another--allowing downbanded cellular systems to become part of national or international cellular telephone networks. Furthermore, using the cellular air interface and conventions for downbanded cellular channels allows downbanded cellular system operators to leverage off of existing cellular technology--dramatically reducing downbanded cellular system development time while providing the latest in cellular telephone features at relatively low cost.
To achieve compatibility with conventional cellular systems, downbanded cellular systems generally must use the same air interface that the conventional cellular systems use. This means using the same channel bandwidth as conventional cellular systems. Most people know that different kinds of radio services often have different channel bandwidths. For example, AM broadcast stations each use a 10 kHz wide channel, while FM broadcast stations take up 20 times that amount of spectrum space. Similarly, cellular telephones generally use 30 kHz wide channels, and cellular telephones and associated equipment have been designed to operate using these channel widths and associated channel spacings. The SMR radio band, in contrast, was long ago divided into channels that are each 25 kHz wide--a bandwidth that is too small to be compatible with widely used cellular air interface requiring 30 kHz channels.
To accommodate existing SMR operators, the FCC is not changing the channelization of the SMR band to 30 kHz. However, the FCC is allowing downbanded cellular operators to operate on the SMR band using 30 kHz wide emissions (or any other technology)--so long as they do not interfere with adjacent channels (and also meet occupied bandwidth requirements). This means that it is possible to use cellular radio equipment designed for 30 kHz channelization on the SMR band--if proper precautions are taken.
A key issue in implementing a downbanded cellular system is the possibility of having an SMR system operating on a 25 kHz channel adjacent to downbanded cellular 30 kHz operation. This situation creates the possibility of adjacent channel interference conditions that are not part of normal cellular operation. Downbanded cellular system operators need an efficient way to come up with a communications system frequency plan that minimizes cochannel and adjacent channel interference (including interference with adjacent SMR channels) while maximizing cell capacity.